Structures of bacterial iron-containing proteins, several functional in bioconversion of solar energy, are being determined. A number of these proteins, available in pure form and bulk quantities, have been crystallized and the resultant preparations found adequate for tertiary structure determinations in high resolution, using a variety of procedures including x-ray crystallographic analysis, and a wide range of modern spectrochemical methodology (ORD, CD, NMR, EPR and IR spectroscopy). These preparations include the variant cytochromes c (cytochrome c2, cytochrome c' and "HIPISP") of a number of purple photosynthetic bacteria, and the algal cytochromes "f" of red and blue-green algae. Complete amino acid sequences for some of these proteins have been determined. Others are now being analyzed preparatory to extension of these structure studies to provide a detailed molecular basis for a comparative biochemistry of iron-containing proteins.